Alkoxyamines represented by formula I, ##STR2## in which R.sub.1 and R.sub.2 are, independently, C.sub.1 to C.sub.20 alkyl groups, are important intermediates for compounds used in pharmaceutical and agricultural applications. Due to the presence of an asymmetric carbon atom, the above alkoxyamines exist as a mixture of enantiomers or optical isomers.
It is known that for compounds that exist as optical isomers, one isomer is typically more active than the other, and sometimes one isomer is exclusively active, depending on its use. By using essentially the active enantiomer, lower effective amounts are needed; thus making the production of the active material more cost effective. In addition, if the compound is known to have an adverse effect on the environment, the environmental impact of the enantiomer can be reduced by using lower amounts.
In general, optically active amines are produced in the form of racemic mixtures. Methods which have been used to obtain the active isomer include asymmetric synthesis, resolution of the enantiomers in racemic mixtures, and selective reaction. When the enantiomers are separated by techniques such as selective reaction or resolution, the inactive enantiomer is recovered along with the active enantiomer. If an outlet cannot be identified for the inactive isomer, it becomes a waste stream. A goal in the industry is to maximize production of the active isomer and minimize the environmental impact of an inactive isomeric waste stream. Therefore methods of limiting the amount of inactive isomer produced is of continuing interest. One method of limiting the amount of inactive isomer has been to convert the inactive isomer to the racemic mixture and then recycle the racemic mixture to a separation process. However, one of the problems encountered in producing racemic mixtures is the production of undesirable by-products.
The following patents provide examples of methods which have been used to produce racemic mixtures of optically active amines.
U.S. Pat. No. 3,954,870 (Fukumaru et al., 1976) discloses racemization of optically active .alpha., .beta.-diphenylethylamine by heating the optically active amine at a temperature between 100.degree. C. and the b.p. of the amine in the presence of Raney nickel under an inert gas such as nitrogen, helium or ammonia. Hydrogen is not desired because it promotes reduction of the amines and the formation of by-products. It is reported that by using a dry Raney nickel, a short reaction time is realized and the formation of undesired by-products is prevented.
U.S. Pat. No. 3,970,700 (Nagase et al., 1976) discloses a method for racemizing optically active amines by contacting optically active amines of the formula below with an alkali metal deposited on a solid carrier, an alkali metal dispersed in a liquid medium or an alkali metal alloy at a temperature of from -10 to 50.degree. C.: ##STR3## wherein *C is an asymmetric carbon atom, R.sub.1 is alkyl, aralkyl or aryl and R.sub.2 is aryl or alkoxycarbonyl, the aryl or aralkyl moiety bearing optionally one or more alkyl or alkoxy groups on the aromatic ring, and R.sub.1 and R.sub.2 are different from each other.
U.S. Pat. No. 4,096,186 (Ichikawa et al., 1978) discloses racemization of optically active aminoalcohol compounds having the following formula: ##STR4## wherein R.sub.1 is an alkyl, cycloalkyl, substituted or unsubstituted phenyl, or non-aromatic heterocyclic group, and R.sub.2 is a hydrogen atom or an alkyl or substituted or unsubstituted phenyl group. The compounds are contacted with ammonia and hydrogen in the presence of a hydrogenation catalyst such as nickel, cobalt, copper and noble metals; cobalt is preferred, especially cobalt alone or with a small amount of a metal or metal oxide, such as iron, manganese, zinc, and cesium, or the corresponding oxides.
DE 2,903,589 (Vitt, 1980) (abstracts from Derwent World Patent Index and Chemical Abstracts) discloses racemization of amines having the general formula: R1--CH(NH.sub.2)--R2 (where R1 and R2 are different alkyl or aralkyl residues, the aralkyl residue optionally substituted in the aromatic ring with one or more alkyl, fluorinated alkyl, alkoxy or OH). The amines are treated with hydrogen in the presence of a hydrogenation catalyst (preferably Raney cobalt or Raney nickel) at temperatures of 80 to 300.degree. C.
U.S. Pat. No. 4,990,666 (Harsy, 1991) discloses racemization of optically active amino alcohols by subjecting the amino alcohols to hydrogen under moderate temperatures (100 to 175.degree. C.) and pressures (10 to 50 atmospheres) while in contact with Raney cobalt. Ammonia is not required. The process can be batch or continuous. The amino alcohols have the following general formulas: ##STR5## wherein R.sub.1 is an alkyl group or cycloalkyl group; each R.sub.2 is the same group selected from hydrogen, alkyl group, or cycloalkyl; R.sub.3 is a hydrogen atom or an alkyl group; R.sub.4 is the same groups as R.sub.1 ; and each R.sub.5 is the same group selected from those equal to R.sub.2.
U.S. Pat. No. 5,847,215 (Ditrich, 1998) discloses racemization of optically active arylalkylamines by reaction of the amine with a ketone followed by treatment of the condensation product with a base such as an alcoholate or an amine.